Polytec PDV-100, Руководство пользователя

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4-4
4 Making Measurements
4.3 Setting the Optimal Stand-off Distance
4.3.1 Coherency between Stand-off Distance and Visibility Maximum
The light source of the PDV is a helium neon laser. This is a mulit-mode laser
in which, depending on the laser cavity length, one or a maximum of two
modes can exist. The laser cavity length can vary caused by small changes in
temperature. Thus the laser changes between the one-mode and the two-
mode state. If two modes exist, interference effects cause the intensity of the
resulting optical signal varying periodically with the stand-off distance.
The diagram in
FIGURE
4.2 shows the signal level depending on the stand-off
distance. In the special case of having two modes with equal magnitude
(black line), you have the strongest loss of signal level if the object is located
inside a visibility minimum. But generally there are two modes of different
magnitudes. In this case the signal level is hardly fluctuating (gray line). If
only one mode exists, the signal level is always maximum, independent from
the stand-off distance (dashed gray line).
The stand-off distances at which the signal level is maximal are called
visibility maxima. The visibility maxima recur every 138 mm corresponding to
the laser cavity length.
In practice, it is not usually necessary to search for the visibility maximum as
the PDV is even sensitive enough to make a measurement close to the
minimum. A visibility minimum is indicated during the warm-up phase by
periodic fluctuation of the optical signal level. If you need an optimal
resolution and sensitivity, you should select a stand-off distance close to the
visibility maxima for the PDV.
Figure 4.2: Coherency between stand-off distance and visibility maximum
Signal
level
Stand-off distance in mm
96 234 372
... +138
2 modes with equal magnitude
2 modes with different magnitude
1 mode
Visibility minimum Visibility maximum